Transversely-excited film bulk acoustic resonator matrix filters

ABSTRACT

Radio frequency filters. A radio frequency filter includes a substrate attached to a piezoelectric plate, portions of the piezoelectric plate forming a plurality of diaphragms spanning respective cavities in the substrate. A conductor pattern formed on the piezoelectric plate includes a plurality of interdigital transducers (IDTs) of a respective plurality of resonators, interleaved fingers of each IDT disposed on a respective diaphragm of the plurality of diaphragms. The conductor pattern connects the plurality of resonators in a matrix filter circuit including a first sub-filter and a second sub-filter, each sub-filter comprising two or more resonators from the plurality of resonators.

RELATED APPLICATION INFORMATION

This patent is a continuation-in-part of application Ser. No.17/121,724, filed Dec. 14, 2020, titled ACOUSTIC MATRIX FILTERS ANDRADIOS USING ACOUSTIC MATRIX FILTERS, which claims priority fromprovisional patent application 63/087,789, filed Oct. 5, 2020, entitledMATRIX XBAR FILTER, both of which are incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a passband or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 use the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77and n79must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is anacoustic resonator structure for use in microwave filters. The XBAR isdescribed in patent U.S. Pat. No. 10,491,291, titled TRANSVERSELYEXCITED FILM BULK ACOUSTIC RESONATOR, which is incorporated herein byreference. An XBAR resonator comprises an interdigital transducer (IDT)formed on a thin floating layer, or diaphragm, of a single-crystalpiezoelectric material. The IDT includes a first set of parallelfingers, extending from a first busbar and a second set of parallelfingers extending from a second busbar. The first and second sets ofparallel fingers are interleaved. A microwave signal applied to the IDTexcites a shear primary acoustic wave in the piezoelectric diaphragm.XBAR resonators provide very high electromechanical coupling and highfrequency capability. XBAR resonators may be used in a variety of RFfilters including band-reject filters, band-pass filters, duplexers, andmultiplexers. XBARs are well suited for use in filters forcommunications bands with frequencies above 3 GHz. Matrix XBAR filtersare also suited for frequencies between 1 GHz and 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view, two schematic cross-sectionalviews, and a detailed cross-sectional view of a transversely-excitedfilm bulk acoustic resonator (XBAR).

FIG. 2A is an equivalent circuit model of an acoustic resonator.

FIG. 2B is a graph of the admittance of an ideal acoustic resonator.

FIG. 2C is a circuit symbol for an acoustic resonator.

FIG. 3A is a schematic diagram of a matrix filter using acousticresonators.

FIG. 3B is a schematic diagram of a sub-filter of FIG. 3A.

FIG. 4 is a schematic diagram of a matrix filter usingtransversely-excited film bulk acoustic resonators.

FIG. 5 is plan view of an embodiment of the matrix filter of FIG. 4.

FIG. 6 is a schematic cross-sectional view of the matrix filter of FIG.5.

FIG. 7 is a graph of input-output transfer functions of an embodiment ofthe multiplexer of FIGS. 4-6.

FIG. 8 is flow chart of a process for making a matrix filter usingtransversely-excited film bulk acoustic resonators.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view, orthogonal cross-sectionalviews, and a detailed cross-sectional view of a transversely-excitedfilm bulk acoustic resonator (XBAR) 100. XBAR resonators such as theresonator 100 may be used in a variety of RF filters includingband-reject filters, band-pass filters, duplexers, and multiplexers.XBARs are particularly suited for use in filters for communicationsbands with frequencies above 3 GHz. The matrix XBAR filters described inthis patent are also suited for frequencies above 1 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. The piezoelectric platemay be Z-cut (which is to say the Z axis is normal to the front and backsurfaces 112, 114), rotated Z-cut, or rotated YX cut. XBARs may befabricated on piezoelectric plates with other crystallographicorientations.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”. In other configurations, the diaphragm 115 may be contiguous withthe piezoelectric plate around at least 50% of the perimeter 145 of thecavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 under the diaphragm 115. The cavity 140 may be formed, for example,by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. The primary acoustic mode of an XBAR is abulk shear mode where acoustic energy propagates along a directionsubstantially orthogonal to the surface of the piezoelectric plate 110,which is also normal, or transverse, to the direction of the electricfield created by the IDT fingers. Thus, the XBAR is considered atransversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the diaphragm 115 ofthe piezoelectric plate which spans, or is suspended over, the cavity140. As shown in FIG. 1, the cavity 140 has a rectangular shape with anextent greater than the aperture AP and length L of the IDT 130. Acavity of an XBAR may have a different shape, such as a regular orirregular polygon. The cavity of an XBAR may have more or fewer thanfour sides, which may be straight or curved.

The detailed cross-section view (Detail C) shows two IDT fingers 136 a,136 b on the surface of the piezoelectric plate 110. The dimension p isthe “pitch” of the IDT and the dimension w is the width or “mark” of theIDT fingers. A dielectric layer 150 may be formed between and optionallyover (see IDT finger 136 a) the IDT fingers. The dielectric layer 150may be a non-piezoelectric dielectric material, such as silicon dioxideor silicon nitride. The dielectric layer 150 may be formed of multiplelayers of two or more materials. The IDT fingers 136 a and 136 b may bealuminum, copper, beryllium, gold, tungsten, molybdenum, alloys andcombinations thereof, or some other conductive material. Thin (relativeto the total thickness of the conductors) layers of other metals, suchas chromium or titanium, may be formed under and/or over and/or aslayers within the fingers to improve adhesion between the fingers andthe piezoelectric plate 110 and/or to passivate or encapsulate thefingers and/or to improve power handling. The busbars of the IDT 130 maybe made of the same or different materials as the fingers.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds ofparallel fingers in the IDT 110. Similarly, the thickness of the fingersin the cross-sectional views is greatly exaggerated.

An XBAR based on shear acoustic wave resonances can achieve betterperformance than current state-of-the art surface acoustic wave (SAW),film-bulk-acoustic-resonators (FBAR), and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices. In particular, the piezoelectriccoupling for shear wave XBAR resonances can be high (>20%) compared toother acoustic resonators. High piezoelectric coupling enables thedesign and implementation of microwave and millimeter-wave filters ofvarious types with appreciable bandwidth.

The basic behavior of acoustic resonators, including XBARs, is commonlydescribed using the Butterworth Van Dyke (BVD) circuit model as shown inFIG. 2A. The BVD circuit model consists of a motional arm and a staticarm. The motional arm includes a motional inductance L_(m), a motionalcapacitance C_(m), and a resistance R_(m). The static arm includes astatic capacitance C₀ and a resistance R₀. While the BVD model does notfully describe the behavior of an acoustic resonator, it does a good jobof modeling the two primary resonances that are used to design band-passfilters, duplexers, and multiplexers (multiplexers are filters with morethan 2 input or output ports with multiple passbands).

The first primary resonance of the BVD model is the motional resonancecaused by the series combination of the motional inductance L_(m) andthe motional capacitance C_(m). The second primary resonance of the BVDmodel is the anti-resonance caused by the combination of the motionalinductance L_(m), the motional capacitance C_(m), and the staticcapacitance C₀. In a lossless resonator (R_(m)=R₀=0), the frequencyF_(r) of the motional resonance is given by

$\begin{matrix}{F_{r} = \frac{1}{2\pi\sqrt{L_{m}C_{m}}}} & (1)\end{matrix}$The frequency F_(a) of the anti-resonance is given by

$\begin{matrix}{F_{a} = {F_{r}\sqrt{1 + \frac{1}{\gamma}}}} & (2)\end{matrix}$where γ=C₀/C_(m) is dependent on the resonator structure and the typeand the orientation of the crystalline axes of the piezoelectricmaterial.

FIG. 2B is a graph 200 of the magnitude of admittance of a theoreticallossless acoustic resonator. The acoustic resonator has a resonance 212at a resonance frequency where the admittance of the resonatorapproaches infinity. The resonance is due to the series combination ofthe motional inductance L_(m) and the motional capacitance C_(m) in theBVD model of FIG. 2A. The acoustic resonator also exhibits ananti-resonance 214 where the admittance of the resonator approacheszero. The anti-resonance is caused by the combination of the motionalinductance L_(m), the motional capacitance C_(m), and the staticcapacitance C₀. In a lossless resonator (R_(m)=R₀=0), the frequencyF_(r) of the resonance is given by

$\begin{matrix}{F_{r} = \frac{1}{2\pi\sqrt{L_{m}C_{m}}}} & (1)\end{matrix}$The frequency F_(a) of the anti-resonance is given by

$\begin{matrix}{F_{a} = {F_{r}\sqrt{1 + \frac{1}{\gamma}}}} & (2)\end{matrix}$

In over-simplified terms, the lossless acoustic resonator can beconsidered a short circuit at the resonance frequency 212 and an opencircuit at the anti-resonance frequency 214. The resonance andanti-resonance frequencies in FIG. 2B are representative, and anacoustic resonator may be designed for other frequencies.

FIG. 2C shows the circuit symbol for an acoustic resonator such as anXBAR. This symbol will be used to designate each acoustic resonator inschematic diagrams of filters in subsequent figures.

FIG. 3A is a schematic diagram of a matrix filter 300 using acousticresonators. The matrix filter 300 includes an array 310 of n sub-filters320-1, 320-2, 320-n connected in parallel between a first filter port(FP1) and a second filter port (FP2), where n is an integer greater thanone. Each of the n sub-filters 320-1, 320-2, 320-n is a bandpass filterhaving a bandwidth about 1/n times the bandwidth of the matrix filter300. The sub-filters 320-1, 320-2, 320-n have contiguous passbands suchthat the bandwidth of the matrix filter 300 is equal to the sum of thebandwidths of the constituent sub-filters. In the subsequent examples inthis patent n=3. n can be less than or greater than 3 as necessary toprovide the desired bandwidth for the matrix filter 300.

The array 310 of sub-filters is terminated at both ends by acousticresonators XL1, XL2, XH1, and XH2, which are preferably but notnecessarily XBARs. The acoustic resonators XL1, XL2, XH1, and XH2 create“transmission zeros” at their respective resonance frequencies. A“transmission zero” is a frequency where the input-output transferfunction of the filter is very low (and would be zero if the acousticresonators XL1, XL2, XH1, and XH2 were lossless). Typically, but notnecessarily, the resonance frequencies of XL1 and XL2 are equal, and theresonance frequencies of XH1 and XH2 are equal. The resonant frequenciesof the acoustic resonators XL1, XL2 are selected to provide transmissionzeros adjacent to the lower edge of the filter passband. XL1 and XL2 maybe referred to as “low-edge resonators” since their resonant frequenciesare proximate the lower edge of the filter passband. The acousticresonators XL1 and XL2 also act as shunt inductances to help match theimpedance at the ports of the filter to a desired impedance value. Inthe subsequent examples in this patent, the impedance at all ports ofthe filters is matched to 50 ohms. The resonant frequencies of acousticresonators XH1, XH2 are selected to provide transmission zeros at orabove the higher edge of the filter passband. XH1 and XH2 may bereferred to as “high-edge resonators” since their resonant frequenciesare proximate the higher edge of the filter passband. High-edgeresonators XH1 and XH2 may not be required in all matrix filters.

FIG. 3B is a schematic diagram of a sub-filter 350 suitable forsub-filters 320-1, 320-2, and 320-n. The sub-filter 350 includes threeacoustic resonators XA, XB, XC connected in series between a firstsub-filter port (SP1) and a second sub-filter port (SP2). The acousticresonators X1, X2, X3 are preferably but not necessarily XBARs. Thesub-filter 350 includes two coupling capacitors CA, CB, each of which isconnected between ground and a respective node between two of theacoustic resonators. The inclusion of three acoustic resonators in thesub-filter 350 is exemplary. A sub-filter may have m acousticresonators, where m is an integer greater than one. A sub-filter with macoustic resonators includes m−1 coupling capacitors. The in acousticresonators of a sub-filter are connected in series between the two portsSP1 and SP2 of a sub-filter and each of the m−1 coupling capacitors isconnected between ground and a node between a respective pair ofacoustic resonators from the in acoustic resonators.

Compared to other types of acoustic resonators, XBARs have very highelectromechanical coupling (which results in a large difference betweenthe resonance and anti-resonance frequencies), but low capacitance perunit area. The matrix filter architecture, as shown in FIG. 3A and FIG.3B, takes advantage of the high electromechanical coupling of XBARswithout requiring high resonator capacitance.

FIG. 4 is a schematic circuit diagram of an exemplary matrix filter 400implemented with XBARs. The matrix filter 400 includes three sub-filters420-1, 420-2, 420-3 connected in parallel between a first filter port(FP1) and a second filter port (FP2). The sub-filters 420-1, 420-2,420-3 have contiguous passbands such that the bandwidth of the matrixfilter 300 is equal to the sum of the bandwidths of the constituentsub-filters. Each sub-filter includes three XBARs connected in seriesand two capacitors. For example, sub-filter 420-1 includes XBARs X1A,X1B, X1C and capacitors C1A, C1B. Components of the other sub-filters420-2 and 420-3 are similarly identified. Low-edge XBARs XL1 and XL2 areconnected between FP1 and FP2, respectively, and ground. All of thecapacitors within the sub-filters are connected to ground through acommon inductor L1. The inclusion of the inductor L1 improves theout-of-band rejection of the matrix filter 400. The matrix filter 400does not include high-edge resonators.

The exemplary matrix filter 400 is symmetrical in that the impedances atFP1 and FP2 are both equal to 50 ohms. The internal circuitry of thefilter is also symmetrical, with XBARs X_A and X_C within eachsub-filter being the same and low-edge resonators XL1 and XL2 being thesame. Other matrix filters may be designed to have significantlydifferent impedances at FP1 and FP2, in which event the internalcircuitry will not be symmetrical.

FIG. 5 is a plan view of an exemplary matrix filter 500 which has thesame schematic circuit diagram as the matrix filter 400 of FIG. 4. Theexemplary matrix filter is an LTE band 41 bandpass filter with apassband from 2496 to 2690 MHz.

The matrix filter 500 includes a Z-cut lithium tantalate piezoelectricplate 510 which is bonded to a substrate (not visible). The thickness ofthe piezoelectric plate is 730 nm. Other matrix filters may use lithiumniobate piezoelectric plates and other crystal orientations includingrotated Z-cut and rotated Y-cut.

The matrix filter 500 includes eleven XBARs, such as the XBAR 520. Acavity (not visible) is formed in the substrate under each XBAR. EachXBAR is shown as a rectangle with vertical hatching and is identified bythe designator (XL1, X1A, . . . ) used in the schematic diagram of FIG.4. The vertical hatching is representative of the direction of the IDTfingers of each XBAR but not to scale. Each XBAR has between 65 and 130IDT fingers. The IDT fingers are aluminum 925 nm thick. The apertures(vertical direction as shown in FIG. 5) of the XBARs range from 40microns to 58 microns, and the lengths (left-right direction as shown inFIG. 5) range from 500 to 1000 microns. In other embodiments of XBARmatrix filters, the XBARs may be divided into sections to limit thelength of the diaphragm within each XBAR. The pitch of the IDTs of eachXBAR is between 7.5 and 8.6 microns and the mark/pitch ratio of eachXBAR is between 0.22 and 0.31.

The XBARs are connected by conductors such as conductor 530.Cross-hatched rectangles are metal-insulator-metal capacitors, of whichonly capacitor 540 is identified. The identified capacitor 540 is C3B inthe schematic diagram of FIG. 4.

Connections from the filter 510 and circuitry external to the filter aremade by means of conductive pads indicated by shaded circles, such asconductive pad 550. The conductive pads for Filter Port 1 (FP1), FilterPort 2 (FP2), and ground (GND) are labeled. The three other conductivepads connect to ground through inductor L1 (in FIG. 4), which is locatedexternal to the filter 510.

As previously described, the sub-filters of a matrix filter havecontiguous passbands that span the passband of the matrix filter. Withina matrix filter, the center frequency of the passband of each sub-filteris different from the center frequency of any other sub-filter.Consequentially, the resonance frequencies of the XBARs in thesub-filter are different from the resonance frequencies of the XBARswithin any other sub-filter.

The resonance frequency of an XBAR is primarily determined by thethickness of the diaphragm within the XBAR. The resonance frequency hasa smaller dependence on IDT pitch and mark or finger width. U.S. Pat.No. 10,491,291 describes the use of a dielectric layer formed betweenthe IDT fingers to adjust the resonance frequency of an XBAR.

FIG. 6 is a schematic cross-section view of the matrix filter 500 at asection plane D-D defined in FIG. 5. The section plane D-D passesthrough one XBAR (X1A, X2A, X3A) from each of the three sub-filters inthe matrix filter 500. Each XBAR includes IDT fingers (of which only IDTfinger 630 is identified) formed on a respective diaphragm spanning arespective cavity in a substrate 620. Each diaphragm includes a portionof a piezoelectric plate 510. As in previous figures, the thickness ofthe piezoelectric plate 510 and the thickness, pitch, and finger widthof the IDTs are greatly exaggerated for visibility. Drawn to scale, thethickness of the piezoelectric plate 610 and the IDT fingers would beless than one-half percent of the thickness of the substrate 620 andeach IDT would have 65 to 130 IDT fingers.

The three detail views illustrate the use of dielectric thickness to setthe resonance frequencies of the XBARs within each sub-filter. Considerfirst the detail view of an IDT finger of XBAR X1A (the middle view ofthe three detail views), which shows an IDT finger 630-1 formed on aportion of the piezoelectric plate 510. The IDT finger 630-1 is shownwith a trapezoidal cross-section. The trapezoidal shape is exemplary andIDT fingers may have other cross-sectional shapes. The IDT finger 630-1and the space between IDT finger 630-1 and adjacent IDT fingers iscovered by a dielectric layer 640-1 having a thickness td1.

Similarly, the right-hand detail shows IDT finger 630-2 from XBAR X2A,which is covered by a dielectric layer 640-2 having a thickness td2. Theleft-hand detail shows IDT finger 630-3 from XBAR X3A, which is coveredby a dielectric layer 640-3 having a thickness td3. The dielectriclayers 640-1, 640-2, 640-3 may be silicon dioxide, silicon nitride,aluminum oxide or some other dielectric material or combination ofmaterials. The dielectric layers 640-1, 640-2, 640-3 may be the same ordifferent materials.

In this example, XBAR X1A is an element of the sub-filter with thelowest passband frequency and XBAR X3A is an element of the sub-filterwith the highest passband frequency. In this case td1>td2>td3≥0.

In a more general case where a matrix filter has n sub-filters, whichare numbered in order of increasing passband frequency, td1>td2> . .. >tdn, where tdi is the dielectric thickness over the XBARs ofsub-filter i.

An XBAR filter device typically includes a passivation dielectric layerapplied over the entire surface of the device, other than contact pads,to seal and passivate the conductor patterns and other elements of thedevice. The thickness t_(pass) of the passivation layer sets the minimumdielectric thickness over the IDTs of the XBARs. Further, a practicalmaximum dielectric thickness over the IDTs of the XBARs is about 0.35times the thickness tp of the piezoelectric plate. Above this dielectricthickness, substantial energy may be coupled into spurious acousticmodes that degrade the performance of a matrix filter. With theseconstraints, 0.35tp≥td1>td2> . . . >tdn≥tpass.

FIG. 7 is graph 700 of the simulated performance of a matrix filtersimilar to the matrix filter 500. The curve 710 is a plot of S21, theinput-output transfer function, of the filter determined by simulationof a physical model of the filter using finite element techniques. Thebroken lines 720 mark the band edges of 5G NR communication band n41.The matrix filter architecture extends the application of XBARs to lowerfrequency communications bands that are impractical using a conventionalladder filter architecture.

Description of Methods

FIG. 8 is a simplified flow chart showing a process 800 for making anXBAR or a filter incorporating XBARs. The process 800 starts at 805 witha substrate and a plate of piezoelectric material and ends at 895 with acompleted XBAR or filter. The flow chart of FIG. 8 includes only majorprocess steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 8.

The flow chart of FIG. 8 captures three variations of the process 800for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 810A, 810B, or 810C.Only one of these steps is performed in each of the three variations ofthe process 800.

The piezoelectric plate may be, for example, Z-cut lithium niobate orlithium tantalate with Euler angles 0, 0, 90°. The piezoelectric platemay be rotated Z-cut lithium niobate with Euler angles 0, β, 90°, whereβ is in the range from −15° to +5°. The piezoelectric plate may berotated Y-cut lithium niobate or lithium tantalate with Euler angles 0,β, 0, where β is in the range from 0 to 60°. The piezoelectric plate maybe some other material or crystallographic orientation. The substratemay preferably be silicon. The substrate may be some other material thatallows formation of deep cavities by etching or other processing.

In one variation of the process 800, one or more cavities are formed inthe substrate at 810A, before the piezoelectric plate is bonded to thesubstrate at 820. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 810A will not penetrate through the substrate.

At 820, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

A conductor pattern, including IDTs of each XBAR, is formed at 830 bydepositing and patterning two or more conductor levels on the front sideof the piezoelectric plate. The conductor levels typically include afirst conductor level that includes the IDT fingers, and a secondconductor level formed over the IDT busbars and other conductors exceptthe IDT fingers. In some devices, a third conductor levels may be formedon the contact pads. Each conductor level may be one or more layers of,for example, aluminum, an aluminum alloy, copper, a copper alloy, orsome other conductive metal. Optionally, one or more layers of othermaterials may be disposed below (i.e. between each conductor layer andthe piezoelectric plate) and/or on top of each conductor layer. Forexample, a thin film of titanium, chrome, or other metal may be used toimprove the adhesion between the first conductor level and thepiezoelectric plate. The second conductor level may be conductionenhancement layer of gold, aluminum, copper or other higher conductivitymetal may be formed over portions of the first conductor level (forexample the IDT bus bars and interconnections between the IDTs).

Each conductor level may be formed at 830 by depositing the appropriateconductor layers in sequence over the surface of the piezoelectricplate. The excess metal may then be removed by etching through patternedphotoresist. The conductor level can be etched, for example, by plasmaetching, reactive ion etching, wet chemical etching, and other etchingtechniques.

Alternatively, each conductor level may be formed at 830 using alift-off process. Photoresist may be deposited over the piezoelectricplate, and patterned to define the conductor level. The appropriateconductor layers may be deposited in sequence over the surface of thepiezoelectric plate. The photoresist may then be removed, which removesthe excess material, leaving the conductor level.

When a conductor level has multiple layers, the layers may be depositedand patterned separately. In particular, different patterning processes(i.e. etching or lift-off) may be used on different layers and/or levelsand different masks are required where two or more layers of the sameconductor level have different widths or shapes.

At 840, dielectric layers may be formed by depositing one or more layersof dielectric material on the front side of the piezoelectric plate. Aspreviously described, the dielectric layers may include a differentdielectric thickness over the IDT fingers of the XBARs within eachsub-filter. Each dielectric layer may be deposited using a conventionaldeposition technique such as sputtering, evaporation, or chemical vapordeposition. Each dielectric layer may be deposited over the entiresurface of the piezoelectric plate, including on top of the conductorpattern. Alternatively, one or more lithography processes (usingphotomasks) may be used to limit the deposition of the dielectric layersto selected areas of the piezoelectric plate, such as only between theinterleaved fingers of the IDTs. Masks may also be used to allowdeposition of different thicknesses of dielectric materials on differentportions of the piezoelectric plate.

The matrix filter shown in FIG. 5 and FIG. 6 includesmetal-insulator-metal (MIM) capacitors. A MIM capacitor consists of afirst metal level and a second metal level separated by a dielectriclayer. When a matrix filter includes MIM capacitor, the steps of formingthe conductor patterns at 830 and forming the dielectric layers at 840must overlap. At least one dielectric layer has to be formed at 840after a first metal level is formed at 830 and before a final metallevel is formed at 830.

In a second variation of the process 800, one or more cavities areformed in the back side of the substrate at 810B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In the second variation of the process 800, a back-side dielectric layermay be formed at 850. In the case where the cavities are formed at 810Bas holes through the substrate, the back-side dielectric layer may bedeposited through the cavities using a conventional deposition techniquesuch as sputtering, evaporation, or chemical vapor deposition.

In a third variation of the process 800, one or more cavities in theform of recesses in the substrate may be formed at 810C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device.

In all variations of the process 800, the filter device is completed at860. Actions that may occur at 860 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at 860is to tune the resonant frequencies of the resonators within the deviceby adding or removing metal or dielectric material from the front sideof the device. After the filter device is completed, the process ends at895.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

The invention claimed is:
 1. A radio frequency filter, comprising: asubstrate; a piezoelectric plate having front and back surfaces, theback surface attached to the substrate, portions of the piezoelectricplate forming a plurality of diaphragms spanning respective cavities inthe substrate; and a conductor pattern formed on the front surface, theconductor pattern including a plurality of interdigital transducers(IDTs) of a respective plurality of resonators, interleaved fingers ofeach IDT disposed on a respective diaphragm of the plurality ofdiaphragms, wherein the conductor pattern connects the plurality ofresonators in a matrix filter circuit comprising a first sub-filter anda second sub-filter, each sub-filter comprising two or more resonatorsfrom the plurality of resonators, wherein the piezoelectric plate andthe plurality of IDTs are configured such that respective radiofrequency signals applied to the IDTs excite respective shear primaryacoustic modes in the respective diaphragms.
 2. The filter of claim 1,further comprising: a first port and a second port, wherein the two ormore resonators of the first sub-filter are connected in series betweenthe first port and the second port, and the two or more resonators ofthe second sub-filter are connected in series between the first port andthe second port.
 3. The filter of claim 2, further comprising: a firstlow-edge resonator, from the plurality of resonators, connected betweenthe first port and ground; and a second low-edge resonator, from theplurality of resonators, connected between the second port and ground,wherein respective resonance frequencies of the first and secondlow-edge resonators are adjacent to a lower edge of a passband of thefilter.
 4. The filter of claim 3, further comprising: a first high-edgeresonator, from the plurality of resonators, connected between the firstport and ground; and a second high-edge resonator, from the plurality ofresonators, connected between the second port and ground, whereinrespective resonance frequencies of the first and second high-edgeresonators are adjacent to a higher edge of the passband of the filter.5. A radio frequency filter, comprising: a substrate; a piezoelectricplate having front and back surfaces, the back surface attached to thesubstrate, portions of the piezoelectric plate forming a plurality ofdiaphragms spanning respective cavities in the substrate; and aconductor pattern formed on the front surface, the conductor patternincluding a plurality of interdigital transducers (IDTs) of a respectiveplurality of resonators, interleaved fingers of each IDT disposed on arespective diaphragm of the plurality of diaphragms, wherein theconductor pattern connects the plurality of resonators in a matrixfilter circuit comprising a first sub-filter and a second sub-filter,each sub-filter comprising two or more resonators from the plurality ofresonators, wherein a first thickness of a first dielectric layer formedover the IDTs of the two or more resonators of the first sub-filter isdifferent from a second thickness of a second dielectric layer formedover the IDTs of the two or more resonators of the second sub-filter. 6.The filter of claim 5, wherein the matrix filter circuit comprises athird sub-filter comprising two or more resonators from the plurality ofresonators, and a third thickness of a third dielectric layer formedover the IDTs of the two or more resonators of the third sub-filter isdifferent from the first thickness and the second thickness.
 7. A radiofrequency filter, comprising: a substrate; a piezoelectric plate havingfront and back surfaces, the back surface attached to the substrate,portions of the piezoelectric plate forming a plurality of diaphragmsspanning respective cavities in the substrate; and a conductor patternformed on the front surface, the conductor pattern including a pluralityof interdigital transducers (IDTs) of a respective plurality ofresonators, interleaved fingers of each IDT disposed on a respectivediaphragm of the plurality of diaphragms, wherein the conductor patternconnects the plurality of resonators in a matrix filter circuitcomprising a first sub-filter and a second sub-filter, each sub-filtercomprising two or more resonators from the plurality of resonators,wherein each of the first and second sub-filters comprises: m resonatorsconnected in series, where m is an integer greater than one; and m-1capacitors, each capacitor connected from a junction between two of them resonators and a common terminal.
 8. The filter of claim 7, whereineach of the m-1 capacitors of each sub-filter are metal-insulator-metalcapacitors.
 9. The filter of claim 7, wherein an inductor is connectedfrom the common terminal to ground.
 10. A radio frequency filter,comprising: a first port and a second port; n sub-filters, eachsub-filter including m resonators connected in series between the firstport and the second port, where n and m are integers greater than one;and a first low-edge resonator connected from the first port to groundand a second low-edge resonator connected from the second port toground, wherein each of the first and second low-edge resonators and them resonators within the n sub-filters comprises: a piezoelectric plate,a portion of the piezoelectric plate forming a diaphragm spanning acavity in a substrate, and an interdigital transducer (IDT) formed on asurface of the piezoelectric plate with interleaved IDT fingers disposedon the diaphragm, the piezoelectric plate and the IDT configured suchthat a radio frequency signal applied to the IDT excites a shear primaryacoustic mode in the diaphragm.
 11. The filter of claim 10, wherein asingle substrate and a single piezoelectric plate are common to all ofthe first and second low-edge resonators and the m resonators within then sub-filters.
 12. The filter of claim 10, wherein the n sub-filters arenumbered in order of increasing passband frequency, dielectric layersare formed over the IDT fingers of each resonator, where tdi is athickness of the dielectric layer formed over the IDT fingers of the mresonators of sub-filter i, and td1>td2>. . . . >tdn.
 13. The filter ofclaim 12, further comprising: a passivation layer having a thicknesstpass formed over the conductor pattern and the piezoelectric plate,wherein tdn≥tpass.
 14. The filter of claim 12, wherein the piezoelectricplate has a thickness tp, and 0.35tp≥td1.
 15. The filter of claim 10,each of the n sub-filters further comprising: m-1 capacitors, eachcapacitor connected from a junction between two of the m resonators anda common terminal.
 16. The filter of claim 15, wherein each of the m-1capacitors of each of the n sub-filters are metal-insulator-metalcapacitors.
 17. The filter of claim 15, wherein an inductor is connectedfrom the common terminal to ground.